Congratulations, those are some great Christmas presents;-).
Enjoy the Fuji E6 kits, but honestly, having used both the Kodak and Tetnal kits, I would continue to use Tetenal even if the Kodak were still available. The results are identical - quality sparkling slides that will last a lifetime. The blix based kits are fine for 1,2,3 shot then discard use. They are not however suitable for replenished systems. Which is no problem at all for home DIY users, as most are not looking to fire up an industrial processor doing many rolls each day (with small replenishment injections) and changing chemicals only evey so many weeks (a replenished system).
I too was concerned at first, but did much web searching and did not find any posts claiming faded slides from Tetenal kits (which have been in use since at least the mid 1990's). Actually, I found the opposite - a few users who have 15 plus year old Tetenal slides that are still pristine. In fact, as Tetenal uses the original and time tested use of formaldehyde based final stabilizing rinse, I would bet that it may be more durable than the reduced formaldehyde process introduced later.
Also, I have not found any issues with silver retention (the other claim made against 3 bath). Again, the blix is mixed, used, and discarded in a week or two, while it is still fresh and potent.
As a final note, washing is critical. I double the wash time between steps, especially the final wash. This should give better longevity than most lab based processing, where they are concerned with balancing profit, throoughput, and longevity (in that order). Residual contaminants can be harmful and increase fading.
Anyways, enjoy the Fuji 6 bath, but for others, dont be disuaded from trying the Tetenal kit. It is now avaialble again in the US through Freestyle Photo, and is key to the long term survival of slide film here as lab afetr lab closes. Well, longer term in a very relative sense....
Plus it gives fantastic results and is super easy to use.